The B&M in Stenhousemuir, near Larbert/Stirling in Scotland is clearing out their Miracle-Gro compost. The 50L bags are on sale for £1.49 each (currently £11.29 on Amazon), while the 20L bags are on sale for £0.69. They had 50+ bags of 50L left, and easily 100+ bags of 20L. Might be in other B&M stores as well, but YMMV.